When a ship is moored alongside a wharf or a quay in a harbor, mooring ropes anchoring the ship must be properly tensioned so as to hold the ship in an appropriate position. If no effort is made to maintain the mooring ropes in correct tension, a hazardous situation might arise for the reason that the mooring ropes will become subjected to greater forces due to the tendency of the ship to move relative to the wharf or quay. There are a number of factors that may make the ship move relative to the wharf or quay. These factors can include, for example, variations of the level of water surface due to the cyclic tidal changes, and variations of the displacement of the ship due to cargo loading and/or unloading. These factors will cause the ship to vary its altitude with respect to the wharf or quay, and hence will vary the tension of the mooring ropes of a given length between the ship and the wharf or quay. Furthermore, the ship might be rocked or rolled by waves or wind to induce a fluctuating tension in the mooring ropes. In a situation in which such movements have great amplitudes, the mooring ropes might fail, resulting in a danger to personnel in the area of the ship and a risk of damage to the ship. The tension of the rope or the torque of the rope is either measured or computed on the basis of other measured variables. It is possible to measure the speed of the motor, the torque of the motor or the torque of the winding drum or the tension of the rope.
EP0676365 discloses a winch having at least one winding drum that is connected to an electrical drive via a gearbox. The electrical drive is an asynchronous alternating current (AC) motor connected to a speed control device and fitted with a brake device. The speed control has a speed indicator for detecting an existing rotational speed. The speed control device is coordinated by a control unit which may be, for example, a programmable controller taking the detected rotational speed and a target value of the rotational speed as inputs.
According to EP0676365, the starting point is important to the control of the winch, since the measured or computed value of the torque is not known for the control system. For instance, the measured value does not give an exact value of the tension of the rope and the torque required on the shaft of the motor and their correlations, because there are gearbox and other losses between the motor shaft and the rope.
Further, the speed indicator or the rope tension indicator is susceptible to hard weather conditions especially when the winch is being used as machinery on an open deck of a ship.